The present invention relates to the field of vehicle position sensing and, more particularly, the present invention relates to sensing an angular position of a vehicle, such as its roll, pitch or yaw.
Owing to their precision and accuracy, Global Navigation Satellite Systems (GNSS) have become the de facto standard for vehicle navigation solutions. However, in automotive applications, the prerequisite of having line-of-sight view of the sky is not always met. For instance, around high-rise buildings, dense foliage, in tunnels and under stacked roads and rooftops, GNSS reception is severely compromised.
Dead reckoning refers to the process of augmenting GNSS position fixes with additional sensor information to deduce the vehicle's position during GNSS outage. One type of sensor that is often used in dead reckoning is the vehicle wheel sensor, which can provide information on the distance travelled when GNSS signals are unavailable. Typically these count the number of wheel revolutions via an axle encoder placed on the wheel axle.
However, dead reckoning position updates based purely on wheel rotation have their limitations because vehicles can move in three dimensions. Take for instance a vehicle travelling a distance L up an incline of 30 degrees, as illustrated in FIG. 1. The wheel encoder would deduce a distance L from the start of the ramp although the actual distance travelled in the horizontal direction is only 0.87 L. Hence, the dead reckoning error is a 13% overestimate in the horizontal direction and no indication is provided of the distance travelled in the vertical direction. Using differential wheel rotation, e.g., a separate encoder on the left rear wheel and another encoder on the right rear wheel, can allow dead reckoning in two dimensions, however, this still does not provide any information about movement in the third dimension.
For this reason, many modern dead reckoning systems employ a host of additional sensors, such as accelerometers and gyroscopes, to detect vehicle movements. Having knowledge of the angular position of the vehicle can greatly improve the position estimate of the vehicle when GNSS signals are unavailable. In the foregoing example, having knowledge of the vehicle's pitch would allow a more accurate determination of the vehicle's position in three dimensions. As discussed in the following, there are various known techniques to determine the pitch of the vehicle.
Gyroscope Only Approach
In principal, a gyroscope perfectly aligned with the traverse axis (y-axis) of the body of a vehicle can be used to determine the vehicle's pitch. Although simple, there are several reasons why pitch determination based only on such a gyroscope measurement is not very accurate.
One issue associated with using gyroscopes is caused by the nature of the sensor itself. Gyroscopes only provide an angular rate and not an absolute measure of the angle. To obtain the latter, the output from the gyroscope needs to be integrated. However without knowledge of the initial conditions, i.e. the initial pitch of the body in which the gyroscope is mounted, the calculated output will be in error unless the initial pitch of the body is zero at t=0.
Secondly, since gyroscopes only indicate a rate of change of angular displacement, the measurements in some circumstances are of extremely short duration. Imagine for instance a vehicle traversing from level ground onto a ramp with a constant slope. The output of a y-axis gyroscope would register the change in pitch angle at the instant the vehicle enters the ramp. However, this measurement would only exist for a very short duration before returning to zero as the vehicle climbs the ramp. The very short duration of the measurement makes it challenging to obtain reliable results, especially if the pitch is used to deduce the vehicle's altitude.
Multi-Sensor Approach
More commonly, gyroscopes are used together with accelerometers for more accurate determination of angular position. In determining the pitch of a vehicle, accelerometers often provide more reliable results. In the case of a vehicle traversing a ramp as discussed earlier, an accelerometer would continuously provide a measurement while travelling up the constant incline even though the pitch hasn't changed. This is because the accelerometer provides a measurement of the accelerations the vehicle is experiencing.
One way of taking advantage of both types of sensors to determine an accurate pitch angle involves calculating the difference of the two angles determined by each type of sensor and feeding back this ‘error’ difference to correct the pitch angle measured by the gyroscope sensor. The output of the gyroscope is integrated as before, except this time it is correlated with information indirectly gathered from the accelerometer sensor.
Accelerometer Only Approach
The pitch of a stationary or non-accelerating vehicle can also be determined using a single accelerometer. For example, an accelerometer sensing longitudinal (x-axis) acceleration of a vehicle parked or moving with constant velocity along an incline can be used to directly deduce the incline angle or pitch of the vehicle (according to AX=1 g×sin(θ), where AX denotes the accelerometer output and θ is the pitch of the vehicle). However, such a determination is only accurate when the sensor is oriented correctly in the vehicle. Otherwise, any rotation about the other axes affects the magnitude of the acceleration sensed along the vehicle's x-axis and thus introduces an error into the pitch calculation. Furthermore, if the vehicle is moving with non-zero acceleration, a means is additionally required to determine the acceleration of the vehicle due to its own forward motion. This extra component of acceleration then needs to be accounted for and subtracted from the acceleration sensed by the accelerometer.
What is needed are improved techniques for determining the angular position of a vehicle.